Last year, a man from Louth purchased a box of tools at his local car boot sale. Picking through them, he came across an oval brass disc that looked older than the other bits and pieces in the box.

It was a large badge, about 5cm high and less than 4cm wide. When he looked closely, he could see the words "Royal Drogheda" engraved across the top and "Infantry" along the base, with the arms of Drogheda on a shield surmounted by a royal crown vaguely visible in the centre of the plate. It was an interesting-looking yoke, but its new owner could find no record of the "Royal Drogheda Infantry". Unsure as to what the object actually was, he took it into Whyte's auctioneers.

The auctioneers recognised it immediately. The brass disc was an Irish Volunteers' cross belt plate, dating from the late 18th century. Designed to fit on to the cross belt of a military uniform, this one had belonged to a member of the corps usually known as the Royal Drogheda Rifles (18th century names were more fluid than they are today). "It was one of those militia regiments that changed names and I don't think anyone had come across that name before," said the auctioneers.

In terms of the value of the piece, its relative obscurity was a good thing. The cross belt-plates, with the belts that came with them, were often the only piece of uniform that the militia men actually had.

Frequently, they provide the only record that these militia existed at all. And the rarer they are, the more they are likely to be worth. The owner of the newly-identified cross belt-plate was quite taken aback when the auctioneers valued it at between €600 and €1,000. It sold on September 17, 2016 for €640.

Although this price represents a good return on a car boot sale purchase, many cross belt-plates are worth much more than this. The Royal Drogheda Infantry example was poorly made and in poor condition. Typically, cross belt-plates were made by silversmiths and the standard of design and craftsmanship tends to be relatively high. Most were made in brass or copper, but the cross belt-plates belonging to the commanding officers were silver (or silvered). These, being rarer, are worth more.

In May 2015, a cross belt-plate from the hitherto almost unknown Myroe Infantry of County Derry (c.1798) fetched €2,400 at Whyte's. At the same auction, a cross belt-plate from the Shelmalier Cavalry of County Wexford (1790s) sold for €1,800. More recently, a 1790s cross belt-plate from the Ovens Union Cavalry sold at Whyte's Eclectic Collector Auction of January 21, 2017 for €750. In the same auction, an engraving after Francis Wheatley's famous painting, The Dublin Volunteers On College Green, 4th November 1779, sold for €360.

In general, even a well-worn cross belt-plate can fetch up to €200, so long as it is genuine. If you have one, first check that it is really engraved (you'll see where the tool has scarred it). Fakes tend to be cast from an original and have much smoother lettering. But don't worry if you can't find a record of the name of the corps - the more obscure the militia, the more valuable the cross plate is likely to be.

Cross belt-plates are a record of a fascinating episode in Irish history - the Volunteering movement of the late 18th century (not to be confused with the Irish Volunteers, a nationalist military organisation formed in 1913). The 18th century Volunteers were local militias raised by the gentry in 1778.

The Volunteers were loyalist and officially Protestant. Their stated purpose was to protect and defend Ireland against the threat of French invasion in the absence of British troops, who had been sent off to deal with the American War of Independence. Very quickly, their role became more complicated. They described themselves as "patriots", a particularly 18th century term which came to include demands for free trade and legislative independence.

In the country, they were organised by local landowners and, in the towns, by occupational organisations like lawyers, bankers or even hairdressers. By 1780, they probably numbered between 30,000 and 40,000 Volunteers, both foot soldiers and cavalry.

The love of dressing up and parading in their (self-financed) uniforms ran deep within the movement. The Volunteer's Companion (1794) lists 154 companies of Volunteers: 114 had scarlet uniforms, 18 blue, six green, one dark green, one white, one grey and one buff. Only a few precious relics of this pageantry remain.

A rare survivor, in the form of a silk swallow-tailed flag of the Costello Volunteers (est €20,000 to €30,000) was withdrawn from Whyte's Eclectic Collector Auction of January 21, 2017 and is currently under negotiation. The flag previously sold for £5,500 at Adam's in 1993.

The two-sided flag is of a form typically carried by light cavalry regiments. The front shows a mounted Volunteer offering his services to Hibernia, beneath Gaelic script "Mo Rish agus mo Thir", which roughly translates as "my king and my country". The reverse shows a gilt Maid-of-Erin harp beneath "Costello Volunteers", within a border of shamrock.

Not much else is known about the Costello Volunteers, also called "MacCostello's Regiment". They were based in Mayo/Roscommon, raised in 1779, commanded by Colonel Charles Costello. The flag is a beautiful thing and its symbolism is indicative of a movement that was, to say the least, complex in its allegiances.

See whytes.ie.

In the salerooms

BONHAMS

Walter Osborne's painting

'When The Boats Come In' by the Irish impressionist painter Walter Osborne (est £100,000 to £150,000) leads Bonhams' 19th century European, Victorian and British Impressionist Art sale on March 1.

Osborne was born in Dublin in 1859, but lived mostly in England between 1884 and 1891. At the time when 'When The Boats Come In' (pictured) was painted, he was living in Rye, a small coastal town in Sussex. Charles O'Brien of Bonhams describes the painting as: "a wonderful example of Osborne's English period. It has many of the details - grazing geese, fishermen and villagers at work, a basket of fish with some of them spilling on to the quay - that the artist loved to include in his paintings." For further details, see bonhams.com.

The 600 lots on offer include the part contents of a period house in Dunboyne, Co Meath, as well as the contents from several other houses. The lots include an oak Welsh Dresser (est €400 to €600); a Regency mahogany sideboard with brass gallery rail (est €400 to €500) and a Victorian mahogany grandfather clock (est €800 to €1,200). For further details, see milltownauctionrooms.com; online viewing and bidding is through ukauctioneers.com.

ADAM'S

The January Sale at Adam's takes place on Sunday and represents a general clearout with its fair share of bargains.

Lots include some solid quality furniture with low estimates: a set of six George III Style inlaid mahogany rail-back dining chairs (est €200-€300) and a range of tables (est €50 to €800). For full details see adams.ie.

ANTIQUES AND VINTAGE FAIRS

A fair of antique and vintage items, Hibernian Antique Fairs, takes place at the Cork International Hotel tomorrow and Sunday. The 30 stands at the fair include jewellery and silver, as well as furniture, fine art, coins and banknotes, glass and china.

MEALY'S

Seán Keating's 'Portrait Of An Aran Woman' and her Children was the top lot in Mealy's Winter sale, almost doubling its upper estimate to sell for €130,000. Other top lots in the sale included a pair of Great Irish Deer antlers spanning a width of almost 10ft and with a reconstructed skull (€7,600). See mealys.ie.